Corrosion is a problem in many refineries, particularly crude oil distillation towers. Conventional methods used to measure and control corrosion inside crude distillation towers are discussed in Bieber et al., "New Methods to Measure Corrosive Conditions Inside Crude Distillation Towers", National Association of Corrosion Engineers, Corrosion '90 Meeting, 1990. In the overhead of a crude unit distillation tower, acidic compounds generally cause corrosion when condensed at the dew point of water. Analysis of the dew point water sample shows that the acidic species present are comprised mainly of hydrochloric acid with small amounts of sulfoxy and various organic acids.
It has been known to use amines as neutralizing agents to decrease the corrosion caused by the acids in the system. It has also been known to use organic filmers to impair the attack of the acids on the metal surfaces. Often, a corrosion control program for the overhead of a crude distillation tower will employ a combination of a filmer and a neutralizer.
Neutralizing amines have been injected into the crude charge to the column, into the overhead reflux return or pumparound return, or directly into the dew point water region of the tower. Neutralizing amines generally have a low molecular weight to form soluble chloride salts to reduce the amount of available hydrochloric acid. When there is less available acid, there is less corrosion. However, insoluble amine hydrochloride salts which are solids under conditions in the column can foul or plug column trays. Care must also be taken when selecting the amine since some amine hydrochloride salts can also be corrosive to tray metallurgy.
Filmers generally have a polar end and a non-polar end. The polar end contains one or more heteroatoms and the non-polar ends generally comprise hydrocarbon chains. It is commonly believed that the filmer chemisorbs to the metal surface when the lone pair of electrons on the heteroatom interacts with the empty d orbital of the metals. The hydrophobic chain of the non-polar end facilitates solubility in the hydrocarbon fluid to which it is added, and also helps after chemisorption to hinder the approach of corrosive species to the metal surface.
The mechanism of inhibitor interaction with the metal surface is usually described by the chemisorption process. However, typical metal surfaces in refinery equipment are usually covered with corrosion products, such as sulfides or oxides of iron, that interact with the metal surface and remain on the surface. The presence of these corrosion products is generally beneficial. For example, if the formation of iron sulfide is slow and the sulfide film formed thereby is persistent and stays on the metal surface, then a barrier to corrosion can be formed. This barrier hinders the further attack of corrosive species to the metal surface.
A filmer can protect the metal surface either by interacting with the iron sulfide, or by interacting with the metal surface directly where iron sulfide protection is inadequate. Thus, a filmer that works well in a "sour" application (where H.sub.2 S is present) may not work well in a "sweet" system (without sulfides).
Corrosion in crude distillation towers generally occurs in the hydrocarbon processing equipment wherein the hydrocarbon can typically include 5 to 10 weight percent water, typically employed for steam stripping and/or water washing distillation overheads, but it is typically free of molecular oxygen. The conditions for corrosion are generally above the boiling point of water, typically from about 100.degree. C. to about 230.degree. C., at slightly acidic conditions, e.g. a pH in the range of 5 to 7. These corrosive conditions are generally found in the upper portion of the column where water condenses. Thus, filming inhibitors are usually injected into the reflux return, a pumparound return, or directly into the dew point region of the tower.
Underdeposit corrosion can occur when corrosive agents are trapped by a surface deposit. Such type corrosion can be inhibited by minimizing the presence of deposit-forming materials. A very commonly found deposit material in hydrocarbon processing equipment is iron sulfide which has been deposited relatively quickly without allowing good molecular packing (as opposed to the slowly formed, structurally stable iron sulfide which can form a protective film as mentioned above). Underdeposit corrosion can, to a large extent, be prevented by inhibiting the deposition of iron sulfide particles.
As another consideration for inhibiting corrosion in crude distillation towers, any added chemical must not adversely affect the properties of the distillate. For example, filming amines can cause low values for the water separation index (modified WSIM; ASTM D3948), particularly if used at high treat rates.
Distillation tower corrosion and its control can further be distinguished from the usual types of corrosion which occur in, for example, oil field equipment. Oxygen corrosion, for example, occurs wherever equipment is exposed to atmospheric oxygen, most frequently in offshore installations, brine-handling and injection systems and in shallow producing wells where air is allowed to enter the well. Many corrosion inhibitors which work well in the relatively mild corrosion environment of oil field equipment are not effective in the more severe corrosion environment of crude distillation towers.
U.S. Pat. No. 3,762,873 to Oude Alink discloses the inhibition of corrosion and scale formation in oxygenated systems such as brines, cooling systems, auto radiator systems, drilling muds, etc. using substituted succinimides. The succinimides are said to be obtained by reacting a hydrocarbon succinate with an amine at a preferred mole ratio from 0.8 to 1 of amine to succinic anhydride.